Comparison between experimental and numerical stratigraphy emplaced by
prograding bedforms with a downstream slip face.
E. Viparelli1, A. Blom2 and C. Ferrer-Boix3 1
Department of Civil and Environmental Engineering, University of South Carolina, USA. viparell@engr.sc.edu 2
Faculty of Civil Engineering and Geosciences, Delft University of Technology, Netherlands. astrid.blom@tudelft.nl 3
Department of Geography, The University of British Columbia, Canada. carles.ferrer@upc.edu In the case of non-uniform sediment the streamwise
progradation of bedforms with a downstream slip face (e.g., clinoforms or Gilbert-type deltas, bars, and dunes) typically results in a fining upward profile. This vertical sorting is the result of several processes that can be summarized as follows. Bedload sediment is deposited forming a wedge on the topmost part of the lee side of the bedform. This mechanism is usually termed grain fall. In the grain fall deposit the finer sediment is deposited downstream. When the static angle of repose is exceeded, a grain flow is initiated, i.e., the wedge collapses and the remobilized sediment avalanches down the lee face. During the grain flow sediment sorting takes place, coarse sediment is deposited near the toe (i.e. deepest part) of the lee face, while fine sediment remains in the upper portion of the lee face. An example of a deposit emplaced by a prograding Gilbert delta (Mecca Formation, California, from http:// acad.depauw.edu/~tcope/SedStruct.html), is shown in Figure 1. Here stratigraphic records of the steep delta front are clearly visible in the deposit.
Figure 1: Gilbert delta deposit.
The formation of this type of deposit in fluvio-deltaic environment is, in general, controlled by the sediment supply to the system, the aggradation rate, and sediment transport processes. Thus, a numerical model that is able to store stratigraphy emplaced by those prograding bedforms is the result of the coupling of sub-models that describe (1) the total sediment mass conservation in the system, (2) the mass conservation of sediment in each grain size range, and (3) the sorting process on the lee face. Each sub-model has a different purpose, in other words a model of type (1) cannot be replaced by a model of type (2) or (3). In particular, total sediment mass conservation models, type (1), predict the rates of channel bed aggradation and/or delta progradation. Type (2) models account for the different mobility of sediment particles of different sizes on the stoss face of the bedforms and on the top of Gilbert deltas and clinoforms. Finally, lee face sorting models, type (3), synthetically describe the grain fall - grain flow mechanism that occurs on the lee faces of the bedforms.
In this study we present the comparison between experimental measurements and numerical predictions of stratigraphy emplaced at laboratory scale by a prograding Gilbert delta. Laboratory experiments were performed in the 12 m long and 0.60 m wide tilting flume at the Hydrosystems Laboratory, University of Illinois, Urbana-Champaign. The sediment was a mixture of sand and pea gravel with geometric mean diameter of 3.39 mm, and geometric standard deviation of 1.8. Sediment and water were fed at a constant rate at the upstream end of the flume, and a prograding Gilbert delta formed. At the end of the experimental run, core samples were collected in different locations of the deltaic deposit to characterize the stratigraphy in terms of spatial variation of the grain size distribution of the deposited sediment. Details on the experiments can be found in Ferrer-Boix et al. (2011). The numerical model was built by means of coupling a delta progradation model, an active layer model for mass conservation of non-uniform sediment on the delta top, a lee face sorting model for the delta front, and a procedure for the storage of stratigraphy. Further details on the numerical model can be found in Viparelli et al. (2011, 2012).
Numerical experiments are conducted to compare laboratory and numerical results in terms of a) longitudinal profiles of the elevation of the delta, b) progradation rates of the delta front, and c) vertical and streamwise variation of the grain size distribution of the sediment. Future work will include the application of the model at field scale and the improvement of the numerical scheme to account for cycles of base level variations.
Acknowledgments
The authors sincerely thank Gary Parker for his support and advice in the multiple early stages of this work.
References
Ferrer-Boix, C. (2011),'River incision due to gravel mining and dam removal. Mathematical and experimental study.', PhD thesis, Departament of Hydraulic, Maritime and Environmental Engineering, Technical University of Catalunya. Viparelli E., A. Blom and G. Parker, 2012, Modeling
Stratigraphy formed by prograding Gilbert- type deltas, ID 365, Proceedings River Flow, September 5-7 2012, San Jose’, Costa Rica.
Viparelli E., A. Blom, and G. Parker, 2011, Numerical prediction of the stratigraphy of bedload-dominated deltas: preliminary results, Proceedings River, Coastal and Estuarine Morphodynamics RCEM2011, Tsinghua University, Beijing, China.
